This invention relates to the casting of steel strip in a twin roll caster.
In a twin roll caster molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed.
When casting steel strip in a twin roll caster the casting pool will generally be at a temperature in excess of 1550° C. and it is necessary to achieve very rapid and even cooling of the molten steel over the casting surfaces of the rolls in order to obtain solidification in the short period of exposure of each point on the casting surfaces to the molten steel casting pool during each revolution of the casting rolls. As described in U.S. Pat. No. 5,720,336 the heat flux on solidification can be dramatically affected by the nature of the metal oxides which are deposited on the casting roll surfaces from the steel slag which forms on the casting pool dulling the casting process. Specifically heat flux on solidification can be greatly enhanced if the metal oxides thus deposited on the casting surfaces are in liquid form at the casting temperature thus ensuring that the casting surfaces are each covered by a layer of material which is at least partially liquid at the solidification temperature of the steel. The oxides solidify with the steel to form oxide inclusions in the steel strip but it is most important that they remain in liquid form at the initial solidification temperature of the steel so that they do not deposit as solid particles on the casting surfaces prior to solidification of the steel and thereby inhibit heat transfer to the molten steel.